1. Field of the Invention
The present invention relates to a method of making a flexographic printing sleeve forme including the steps of formation of a sleeve body by providing one or more at least partially cured uniform layers on a sleeve carrier followed by imagewise formation of a relief image on the sleeve body by inkjet printing.
2. Description of the Related Art
Flexography is today one of the most important processes for printing and commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Packaging foils and grocery bags are prominent examples. Coarse surfaces and stretch films can only be economically printed with flexography, making it indeed very appropriate for packaging material printing.
Analogue flexographic printing formes are prepared from printing forme precursors including a photosensitive layer on a support or substrate. The photosensitive layer typically includes ethylenically unsaturated monomers or oligomers, a photo-initiator and an elastomeric binder. The support preferably is a polymeric foil such as PET or a thin metallic plate. Imagewise crosslinking of the photosensitive layer by exposure to ultraviolet and/or visible radiation provides a negative working printing forme precursor which after development with a suitable developer (aqueous, solvent or heat development) leaves a printing relief, which can be used for flexographic printing. Imaging of the photosensitive layer of the printing forme precursor with ultraviolet and/or visible radiation is typically carried out through a mask, which has clear and opaque regions. Crosslinking takes place in the regions of the photosensitive layer under the clear regions of the mask but does not occur in the regions of the photosensitive layer under the opaque regions of the mask. The mask is usually a photographic negative of the desired printed image. The analogue preparation of flexographic printing formes has as major disadvantages the time consuming production of a mask and the poor dimensional stability of the masks with changing environmental temperatures or humidities, making it sometimes unsatisfactory for high quality printing and colour registration. Moreover, the use of separate masks implies consumption of additional consumables and chemistry, with a negative impact on the economical and ecological aspects of the production process, which are often more a concern than the additional time required for making the masks.
Digital imaging, using laser recording, of flexographic printing forme precursors, eliminating the necessity of using a separate mask, is becoming increasingly important in the printing industry. The flexographic printing forme precursor is made laser sensitive by providing e.g. a thin, for UV and visual radiation opaque, infrared (IR) sensitive layer on top of the photopolymerizable layer. Such a flexographic printing forme precursor is typically called a “digital” or “direct-to plate” flexographic printing forme precursor. An example of such a “direct-to-plate” flexographic printing forme precursor is disclosed in EP-A 1 170 121. The thickness of the IR-ablative layer(s) is usually just a few μm. The IR-ablative layer is inscribed imagewise using an IR laser, i.e. the parts the laser beam is incident on are ablated and removed. The actual printing relief is produced in the conventional manner: exposure with actinic light (UV, visible) through the mask, the mask being imagewise opaque to the crosslinking inducing light, resulting in an imagewise crosslinking of the photopolymerizable layer, i.e. relief forming layer. Development with an organic solvent, water or heat removes the photosensitive material from the unexposed parts of the relief forming layer and the residues of the IR-ablative layer. Development may be performed using different developing steps or a single developing step. Since this method still requires a developing step, the improvement in efficiency for producing flexographic printing formes is limited.
In the direct laser engraving technique for the production of flexographic printing formes, a relief suitable for printing is engraved directly into a layer suitable for this purpose. By the action of laser radiation, layer components or their degradation products are removed in the form of hot gases, vapours, fumes, droplets or small particles and nonprinting indentations are thus produced. Engraving of rubber printing cylinders by lasers has been known since the late 60s of the last century. However, this technique has acquired broader commercial interest only in recent years with the advent of improved laser systems. The improvements in the laser systems include better focusing ability of the laser beam, higher power, multiple laser beam or laser source combinations and computer controlled beam guidance. Direct laser engraving has several advantages over the conventional production of flexographic printing plates. A number of time consuming process steps, such as the creation of a photographic negative mask or development and drying of the printing plate, can be dispensed with. Furthermore, the sidewall shape of the individual relief elements can be individually designed in the laser engraving technique.
The methods described above to prepare a flexographic printing forme are all subtractive methods, i.e. non printing areas are removed during wet or dry processing or by laser engraving. Inkjet printing provides an additive method to prepare a flexographic printing forme. For example EP-A 1 428 666 and EP-A 1 637 322 disclose a method of preparing a flexographic printing forme wherein a curable fluid is jetted on a support or substrate having an ink receiving surface. Advantages of such a method of preparing a flexographic printing forme are the absence of any processing steps and the consumption of no more material as necessary to form a suitable relief image (i.e. removal of non printing areas is no longer required).
Conventional flexographic printing formes are “flat” plates. There are however particular applications requiring the use of continuous cylindrical formes, the latter typically referred to as sleeves. These sleeves, in particular seamless sleeves, enable continuous printing and provide improved registration accuracy and shorter change-over-times on press. Furthermore, such sleeves may be well-suited for mounting on laser exposure equipment, where it can replace the drum or be mounted on the drum for exposure by a laser. Continuous printing has applications in the flexographic printing of continuous designs in wallpaper, decoration, gift wrapping paper and packaging.
Sleeves or sleeve bodies are typically made by coating or mold casting an elastomeric layer onto a polymeric or metallic cylinder, a so called basic sleeve, raw sleeve or sleeve carrier. To obtain a uniform surface of the sleeve body, grinding and/or polishing of the sleeve body is necessary to obtain good printing results.
In the present application a sleeve body is a sleeve carrier provided with one or more at least partially cured layers. A sleeve forme is obtained upon forming a relief image on the sleeve body.
A disadvantage of an inkjet method for preparing flexographic printing sleeve formes by jetting the relief image directly onto the sleeve carrier, may be (i) a poor adhesion of the relief image, possibly resulting in a poor runlength and (ii) removal of the relief image after printing, e.g. by mechanical grinding, to reuse the sleeve carrier, becomes difficult without damaging the substrate. The latter, removal of the relief image, is especially important when sleeves are used, since sleeves are expensive.
Applying both a so-called “elastomeric floor” layer on the sleeve carrier in a thickness of typically between 100 μm and several millimeters and the relief parts with an inkjet method, to avoid the two mentioned disadvantages, would be very time consuming.
As an alternative to build the “elastomeric floor”, a typical photopolymer sleeve may be used. Such a photopolymer sleeve typically includes a sleeve carrier and at least one photocurable layer. Typically, when using “digital” imaging to prepare the printing formes, as described above, the “elastomeric floor” layer is established by an overall exposure through the backside of the support while the relief image is realized by imagewise exposure through a mask layer. In an inkjet method, one could cure the complete layer of the photopolymer sleeve, followed by forming the relief image on the completely cured photolayer by inkjet, as suggested in EP-A 1 637 322. However, since the photolayer of a typical photopolymer sleeve is intended to form both the “floor” layer and the relief layer, the “floor” layer realized in this method would be too thick.
It would be advantageous to provide sleeve bodies, including one or more at least partially cured layers, specifically designed to enable a relief image to be formed on it by an inkjet method.
To avoid large stocks of different sleeve bodies, and to enable a high flexibility in choosing the optimum “elastomeric floor” in view of the relief image to be built upon it by inkjet, a method wherein the sleeve body is prepared by applying one or more at least partially cured layers, followed by forming the relief image on it by inkjet, without the need to polish and/or grind the sleeve body, would be highly advantageous. Moreover, it would be particularly advantageous if both providing the sleeve carrier with a dedicated “elastomeric floor” and forming the relief image by inkjet can be performed close to the press, to ensure short access times.
The unpublished EP-A 06 120 823 (filed 18, Sep. 2006) discloses a coating device, with limited floor space requirements and supporting a wide range of sleeve carriers, capable of coating a single or a multitude of uniform layers of direct laser engraveable material, without the need for a grinding and/or polishing step.